1998 Buick Century Loud Bubbling Noise

Tonight when I got home from work - I pulled into the garage and turned off the car. Before I got out of the car I could hear a loud noise - it reminded me of a pan of water boiling on the stove - loud bubbling= banging noise. I lifted the hood but could not see anything. After it cooled down I checked the coolant which was at the normal level, oil normal level. I did not notice it overheating when I was driving but checked it when I moved it out to the driveway, it was only one mark past the middle mark. My a/c is out and of course we are having hot weather. Could it be air trapped in the core?
I was filling my coolant resovoir all the way, but I ran out of coolant, and haven't bought more yet. Could over filling the resovoir cause a problem? Do you have an idea of what I should do or take it to someone?Thank you for your help. Deb